GB687427A - An improved instrument for measuring temperature, air velocities, rate of cooling and the thermal conductivity of gases - Google Patents

Abstract

687,427. Electric indicating systems. NATIONAL COAL BOARD, and' WATERS, P. L. June 21, 1951 [June 27, 1950]. No. 16002/50. Class 40 (i). A temperature measuring bridge circuit. has adjacent variable arms respectively comprising a thermistor a in series with a milliammeter b, and a variable resistance d in series with plural fixed range resistances c<SP>1</SP> c<SP>5</SP> selectible by switch e; and adjacent fixed resistance arms f, g. A galvanometer h shunted by fixed resistances j<SP>1</SP>, j<SP>2</SP> selectible by switch i is connected across the diagonal and a variable potential from a battery k is supplied to the bridge from a series network comprising a main switch p, potentiometer l, resistances m, n and potentiometer o; the bridge being connected between the slider of e, and switch arm r alternately connectible to the slider of o or the junction q of m and n. In temperature measurement the bridge is supplied from q, the galvanometer is appropriately shunted and the bridge rebalanced by selection of the appropriate range resistance and adjustment of resistance d, which is calibrated in temperatures. Alternatively the temperature is indicated by the calibrations of the galvanometer unbalance current. The instrument is applicable to the estimation of carbon monoxide by measuring the heatevolved on oxidation with e.g. mercuric oxide. Humidity is determinable from temperature readings of wet or dry thermistors. For gas velocity measurement the bridge is energized from potentiometer o such that a heating current of larger value flows in the thermistor, and is balanced with the thermistor shielded from air currents. The thermistor is then inserted in the gas current, and the bridge is rebalanced, the necessary change in the balance arm being a measure of gas velocity (which is also indicated by the unbalance current indicated by the calibrated galvanometer). For measuring the rate of cooling of a body the bridge supply voltage is adjusted so that the thermistor is heated in a static atmosphere to the body temperature, the bridge is balanced, and the thermistor current is noted; after which it is exposed to the same cooling conditions as the body. Supply voltage is readjusted until the bridge is again rebalanced, and thermistor current measured. The rate of cooling of the body is then proportional to the difference between the squares of the thermistor currents. The thermal conductivities of gases may be measured by reference to the electrical energy indicated by the. milliammeter, which is the energy required, to compensate for heat dissipation from the thermistor surrounded by the gas, and to rebalance the bridge. Specification 558,299 is referred to.